A inertial frame is a frame of reference that is moving at constant velocity. In an inertial frame, Newton's first law holds.

To study how the laws of physics look to observers in relative motion, we imagine two inertial frames, which we can designate by, say, F and F'.

We assume they are in what is known as standard configuration. This means the frame F' is moving in the x direction at constant velocity v relative to frame F. The y and z axes are the same for both observers.

In pre-relativity physics, there is uniform passage of time throughout the universe for everyone everywhere. Thus, we use the same coordinate for observers in both frames.

Here are 3 laws in Special Relativity. Perhaps postulate would be more appropriate.

1: the laws of physics are the same in all inertial reference frames.

2: All observers in inertial frames will measure the same speed of light, regardless of their state of motion.

3. A particle at rest or with constant velocity in one inertial frame will be at rest or have constant velocity in all inertial frames.

Is that good enough a description?.

I began studying relativity recently. By no means do I want to sound arrogant to those on this site who are learned in the topic. I hoope my description is adequate and accurate.